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Help, Bedford OY servo problems


jim fl4
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Oh the joys for of a servo assisted braking system!  I have rebuild the servo, refitted it and, yup, its not operating.  I think I must have got the valve setting up wrong, I have a good vacuum to it.  I hear the setting up of the valves is super critical and I was hoping someone here may be able to offer its tips??? I think the servo of QL's are the same. thanks, Jim.

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Going from memory they were a Clayton Dewandre unit fitted to Bedfords.They look the same but they have different size hyd master cylinders. Check part numbers.  If you press the brakes with the engine ticking over you will a slight increase in revs then it will settle back down. There is a very noticable difference between brakes with and without servo assistance. If you do an internet search for Clayton Dewandre you should turn up a diagram and some information.

Edited by john1950
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1 hour ago, jim fl4 said:

Oh the joys for of a servo assisted braking system!  I have rebuild the servo, refitted it and, yup, its not operating.  I think I must have got the valve setting up wrong, I have a good vacuum to it.  I hear the setting up of the valves is super critical and I was hoping someone here may be able to offer its tips??? I think the servo of QL's are the same. thanks, Jim.

Jim,

That little Bedford manual you spoke about in another thread, if it is the maintenance book, it has full details of setting the valve adjustment. It has to be done exactly as it says in the book, if not then you have to remove the servo again so you can reset it. It is a fine adjustment and when correct it works very well.

regards, Richard

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Indeed, as Richard explains the setting are crucial . Not all the instruction manuels have the same settings , for my Ql I used the settings out of a original workshop manual.

Good luck .

Guy Labbe 

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Thanks Guy and Richard,  Agree its a very fine adjustment.  I have the little book, but I find it hard to understand exactly where you measure etc.  Anyhow I have removed it to the bench again, and studied how it works.  I'm going to refit it tomorrow.

  Out of interest when I had the engine ticking over and with the top cover over the gearbox off I separated the steel vacuum pipe from the servo and instantly the engine surged...put my thumb over it and the engine purred again.  I could feel the vacuum from this pipe on my thumb.  It was not a killer vacuum, I would say it was no stronger at all than a hoover, does it sound right? because ive no idea what sort of vacuum is normal!

Edited by jim fl4
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3 hours ago, john1950 said:

Going from memory they were a Clayton Dewandre unit fitted to Bedfords.They look the same but they have different size hyd master cylinders. Check part numbers.  If you press the brakes with the engine ticking over you will a slight increase in revs then it will settle back down. There is a very noticable difference between brakes with and without servo assistance. If you do an internet search for Clayton Dewandre you should turn up a diagram and some information.

Thanks John,  Yes its a Dewandre,  On stripping the servo found the vacuum piston seized so I hope that's the reason for the low effect of the brakes.  Having felt the vacuum created it surprises me that such a relatively weak vacuum could multiple the brake effect so much but I assume it must.  

Edited by jim fl4
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45 minutes ago, jim fl4 said:

Having felt the vacuum created it surprises me that such a relatively weak vacuum could multiple the brake effect for much but I assume it must.  

Don't forget that you are only feeling the vacuum over the diameter of the pipe. Multiply that force by the extra area of the piston and then by the length of stroke of the piston devided by the much smaller stroke of the master cylinder and it adds up to a lot more force. Anyway it is certainly going to work better with the piston able to move !

David

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Going back a long way in my memory, the servo effort is atmospheric pressure on one side of the piston, and the depression caused on the other side by opening it to the suction from the manifold or Auto Vac.. Braking effort is dependant upon your relation to sea level and the change in atmospheric pressure. You get the best effort from these systerms going down hill with your foot off the throttle closing the butterfly in the carbureter creating a continuous vacuum. Or by fitting a vac tank but if you have a tank fitted you must fit a vacuum gauge. Petrol injection is no friend of this type of braking system.

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Hi Folks,

Today I out was out in the shed early before it got too hot and after making a slight alteration to the setting of the linkage refitted the unit and success, works perfect and yes it is indeed surprising the effect the servo has on the braking effect.  The engine now has just  a momentary surge (almost  imperceivable) before resuming a steady tick over before a 'hiss' when the brake pedal is released.

I big thank you to all who contributed to educating me with their knowledge.

I'm all ready now for the trucks big outing to the YWE!

All best,  Jim.

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32 minutes ago, jim fl4 said:

Hi Folks,

Today I out was out in the shed early before it got too hot and after making a slight alteration to the setting of the linkage refitted the unit and success, works perfect and yes it is indeed surprising the effect the servo has on the braking effect.  The engine now has just  a momentary surge (almost  imperceivable) before resuming a steady tick over before a 'hiss' when the brake pedal is released.

I big thank you to all who contributed to educating me with their knowledge.

I'm all ready now for the trucks big outing to the YWE!

All best,  Jim.

Well done Jim!

Once you understand how it works it makes it obvious when setting it up. On my QL the brakes are as effective as a vehicle half its age, not bad for a 75 year old lorry.

regards, Richard

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Slight increase in revs is the engine using the air from the servo, you have just let it have when you pressed the brakes. I always thought brakes on a QL was life in reverse, firstly when they are cold, steady and calm. Then the in between years a little bit adventurous, and finally when they get warmed up, kids on a slide wee this is fun at least I have something to press to hold me in the seat.

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2 hours ago, john1950 said:

 I always thought brakes on a QL was life in reverse, firstly when they are cold, steady and calm. Then the in between years a little bit adventurous, and finally when they get warmed up, kids on a slide wee this is fun at least I have something to press to hold me in the seat.

You must have driven a bad QL that's all I can say, I have had mine for 30 years and have been working on lorries and plant for over 50 years so can compare with other types of that era.

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I have driven several, the last one was a good old vehicle, went through several incarnations. It had an S type rear axle at one stage and then a Thornycroft it was always heavy I will post a picture one day. My father was always altering it. Every time the brakes got better, wider back shoes were a bonus. Got a lot of equipment out that was stuck. Vehicles of that era were probably much of a muchness Times were slower then. I suppose compared to a WOT6 with there cables they could be classed as good.

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Interesting comments. Certainly times were much slower, my dad talks of, when as a lad, of  being able to grab hold of the back of lorries to get a free tow up a hill on his bicycle, they were that slow.

Its the same with brakes, of course we cannot judge them against their modern counterparts.

 

 

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I remember that happening on more than once when travelling in his Matador towing a trailer cyclists would grab the back of the trailer as we went past and get a tow up the hill. I used to sit on the engine cover and look out of the back sliding window. A rolling road is a good judge if it is not wet.

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How did you extract the piston in the first place? I spent over an hour this afternoon trying to dismantle a spare - I cannot get the piston to move , and of course you can't get at the other end to tap it out with any sort of drift.

 

Mike

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28 minutes ago, mike30841 said:

How did you extract the piston in the first place? I spent over an hour this afternoon trying to dismantle a spare - I cannot get the piston to move , and of course you can't get at the other end to tap it out with any sort of drift.

 

Mike

Mike,

I recently had the same situation with a spare servo that had been outside for years. Because the piston would not move it was difficult to disconnect the linkage. I took out the spring from the piston and removed the nut in the centre, the plate then came away and prised the old leather piston seal away from the bore and was able to knock out the piston with a long drift which slid inside the piston from the linkage end. Copious amounts penetrating fluid help as well.

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